Patients with severe iliac-obstructive post-thrombotic syndrome (SIO-PTS), a rare complication of lower extremity deep vein thrombosis (DVT), experience profound disability and quality of life (QOL) impairment. Because there is a lack of evidence-based therapy for SIO- PTS, affected patients tend to have their condition either neglected or managed by poorly- justified local opinion. Our long-term goals are two-fold: 1) to validate effective therapy that can reduce PTS symptoms and restore our unfortunate patients with SIO-PTS to better health, limb function, and QOL; and 2) to extend the range of expertise within the clinical trial infrastructure we developed for an ongoing NHLBI-sponsored trial of catheter-based therapy for acute DVT (the ATTRACT Trial) to be able to address challenges in the treatment of chronic DVT and PTS. Our main objective of the current application for a U34 Clinical Trial Planning Grant is to refine and finalize the development of the clinical research protocol and trial capability that are needed to launch the Chronic Venous Thrombosis: Relief with Adjunctive Catheter-Based Therapy (C-TRACT) Study, a multicenter randomized controlled trial that will rigorously determine if the benefits of an innovative strategy of imaging-guided endovascular treatment for patients with established SIO-PTS outweigh the risks and costs. By combining diligent planning, expertise with advanced methodological and biostatistical trial designs (provided by the U24 Clinical Trials Development Resource for Hematologic Disorders), and systematic efforts to engage community partners, we expect to create A) an outstanding clinical trial protocol; B) the potential to rapidly enroll and complete the proposed study; and C) multidisciplinary research groups that can ultimately serve as regional leaders in driving best clinical practices and future research studies to address the needs of patients with chronic DVT and PTS.